An apparatus for perfusion of isolated rat hearts has been developed which circulates a buffered blood analog. This device is used to investigate biochemical and physiological parameters of the heart including blood pressure, pulse pressure, cardiac output, tissue pH and others. The system allows investigators to induce temporary cardioplegia, during which pharmacologic agents may be introduced into the modes of operation: working heart, ischemia (both warm and cold) and Langendorf aortic perfusion. Comparing the pre- and post-plegic parameters allow the investigators to assess the ability of certain drugs and procedures to sustain cellular life through ischemic periods. The results gained from the isolated heart experiments will be tested in large animals in vivo, using the same parameters along with animal survival after surgery. Engineering refinements in the perfusion circuit described above are completed and the system is fully operational.